The present invention relates to apparatus for sensing the torque applied to a shaft.
Torque is often measured by the same general techniques used to measure other types of structural stress. The most common electrical method of measuring stress is to bond a strain sensitive element, such a piezoelectric strain gauge, directly to the member which will experience the strain to be measured. This technique is not favored in situations in which the stressed member is to be rotated or otherwise moved, however, since it is troublesome to electrically interconnect the strain gauge (which will move) with measurement or display circuits (which will usually remain stationary). In such situations, the stress sensor should preferably be stationary, also, and thus should be capable of measuring stress without being rigidly affixed to the movable member. Noncontact stress sensors have been developed to meet this need.
One class of noncontact stress sensor utilizes the known functional dependence of the magnetic permeability of some ferromagnetic materials upon the degree of stress applied to the material. In general, such sensors operate by generating a magnetic field adjacent a ferromagnetic member whose stress state is to be measured, and by then determining the amplitude of the resulting magnetic flux in the member. The level of flux flowing through the material will be directly related to the magnetic permeability of the material, hence the measured magnetic flux level will increase or decrease depending upon the level of stress applied to the member. The sensor elements which generate and measure the magnetic flux are disposed close to but not in contact with the ferromagnetic member. The sensor elements can therefore remain stationary even when the member, itself, moves.
Torque sensors utilizing the functional relationship between magnetic permeability and stress have been developed for automotive applications. A survey of various types of such magnetic based, noncontact torque sensors is found in SAE (Society of Automotive Engineers) Papers 820904 and 820206. The magnetic-based torque sensors described in these papers can measure torque in a moving shaft, and exhibit both rapid response rates and good stability. In the described torque sensors, however, the measured magnetic flux is dependent upon more than just the magnetic permeability of the shaft. Thus, measured magnetic flux is also dependent upon the size of the air gap between the poles of the torque measuring device and the shaft adjacent to which the poles are placed. Translational movement of the shaft, e.g. vibration and/or axial or transverse shifts in the position of the shaft, may cause the air gaps to either increase or decrease in width, thereby introducing spurious changes in sensor output unrelated to actual torque changes within the shaft. In the past, such problems have been dealt with by carefully machining the shaft so as to have a constant radius, and by minimizing the vibrations and shaft movements introducing the gap variations in the first place.